1. Field of the Invention
The present invention relates to a semiconductor device comprising a circuit composed of thin film transistors (hereinafter referred to as TFTs) and a manufacturing method thereof, and for example, an electro-optical device which is represented by a liquid crystal display panel, an EL (electro luminescence) display device, an EC display device, and the like, and an electronic device in which such an electro-optical device is mounted as a part.
Note that, in this specification, the semiconductor device represents all devices which can operate utilizing a semiconductor characteristic, and thus an electro-optical device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
Recently, a development of a semiconductor device with a large area integrated circuit formed by a thin film transistor (TFT), which is formed using a semiconductor thin film (several to several hundreds of nm in thickness) formed on a substrate with an insulating surface, has been progressing. An active matrix liquid crystal display device, an EL display device, and a contact type image sensor are known as typical examples. In particular, Since a TFT using a crystalline silicon film (typically, a polysilicon film) as an active layer (hereinafter referred to as polysilicon TFT) has a high field effect mobility, it can be also used for forming various functional circuits.
For example, in the active matrix liquid crystal display device, a pixel circuit for performing image display in respective functional blocks and a driver circuit (composed of a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit, and the like, which are based on a CMOS circuit) for controlling the pixel circuit, are formed on a single substrate.
In the pixel circuit of the active matrix liquid crystal display device, the TFT (pixel TFT) is disposed in each of several tens to several millions of pixels, and a pixel electrode is provided in the respective pixel TFTs. A counter electrode is provided at the side of a opposing substrate with liquid crystal between the pixel electrode and the counter electrode. Thus, a kind of a capacitor using liquid crystal as a dielectric is formed. Then, a voltage applied to each of the pixels is controlled by a switching operation of the TFT. Thus, the liquid crystal is driven by controlling control an amount of charges to the capacitor, and an amount of transmtted light is controlled to display an image.
The pixel TFT is an n-channel type TFT, and is used as a switching element for applying a voltage to the liquid crystal and driving it. Since the liquid crystal is driven by an alternating current, a system called a frame inversion drive is employed in many cases. In this system, in order to keep consumption power low, it is important that an off-current value (drain current flowing when the TFT is in an off state) is sufficiently kept low as a characteristic required for the pixel TFT.
A low concentration drain (lightly doped drain: LDD) structure is known as a TFT structure for reducing the off-current value. It is required for forming this structure that an impurity element is added with a low concentration to a region between a channel forming region and a source region, or the channel forming region and a drain region, to which the impurity element is added with a high concentration, and the region is referred to as an LDD region. Further, a so-called GOLD (gate-drain overlapped LDD) structure, in which the LDD region is disposed to overlap with a gate electrode through a gate insulating film, is known as a means for preventing a reduction in an on-current value due to a hot carrier. It is known that such a structure relaxes a high electric field near the drain region to prevent a hot carrier injection and thus is effective to prevent a degradation phenomenon.
Here, there is a problem that the off-current value becomes larger than that in the general LDD structure although the GOLD structure has a large effect for preventing the degradation of the on-current value. Thus, this GOLD structure is not preferred for applying the pixel TFT. On the other hand, although the general LDD structure has a large effect for suppressing an increase of the off-current value, an effect for relaxing concentration of an electric field near the drain region to prevent the degradation due to a hot carrier injection is small. Therefore, in the semiconductor device which has a plurality of integrated circuits, such as the active matrix liquid crystal display device, the above problems are made clear particularly in the crystalline silicon TFT as the characteristics are improved a performance required for the active matrix liquid crystal display device are improved.
Conventionally, when the TFT with the LDD structure or the TFT with the GOLD structure is formed, there is a problem that these manufacturing processes become complicated and the number of processes is increased. It is apparent that the increase in the number of processes causes not only an increase of a manufacturing cost, but also a reduction in a manufacturing yield.